1. Field of the Invention
The invention is related to the field of magnetic disk drive systems, and in particular, to providing improved in-stack biasing of the free layer of a magnetoresistive (MR) read element.
2. Statement of the Problem
Many computer systems use magnetic disk drives for mass storage of information. Magnetic disk drives typically include one or more recording heads (sometimes referred to as sliders) that include read elements and write elements. A suspension arm holds the recording head above a magnetic disk. When the magnetic disk rotates, air generated by the rotation of the magnetic disk causes an air bearing surface (ABS) side of the recording head to ride a particular height above the magnetic disk. The height depends on the shape of the ABS. As the recording head rides on the air bearing, an actuator moves an actuator arm that is connected to the suspension arm to position the read element and the write element over selected tracks of the magnetic disk.
To read data from the magnetic disk, transitions on a track of the magnetic disk create magnetic fields. As the read element passes over the transitions, the magnetic fields of the transitions modulate the resistance of the read element. The change in resistance of the read element is detected by passing a sense current through the read element and then measuring the change in voltage across the read element. The resulting signal is used to recover the data encoded on the track of the magnetic disk.
The most common type of read elements are magnetoresistive (MR) read elements. One type of MR read element is a Giant MR (GMR) read element. GMR read elements using only two layers of ferromagnetic material (e.g., NiFe) separated by a layer of nonmagnetic material (e.g., Cu) are generally referred to as spin valve (SV) elements. A simple-pinned SV read element generally includes an antiferromagnetic (AFM) layer, a first ferromagnetic layer, a spacer layer, and a second ferromagnetic layer. The first ferromagnetic layer (referred to as the pinned layer) has its magnetization typically fixed (pinned) by exchange coupling with the AFM layer (referred to as the pinning layer). The pinning layer generally fixes the magnetic moment of the pinned layer perpendicular to the ABS of the recording head. The magnetization of the second ferromagnetic layer, referred to as a free layer, is not fixed and is free to rotate in response to the magnetic field from the magnetic disk. The magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS in response to positive and negative magnetic fields from the rotating magnetic disk. The free layer is separated from the pinned layer by the spacer layer, which is nonmagnetic and electrically conducting.
Another type of spin valve read element is an antiparallel pinned (AP) spin valve read element. The AP-pinned spin valve read element differs from the simple pinned spin valve read element in that an AP-pinned structure has multiple thin film layers forming the pinned layer instead of a single pinned layer. The AP-pinned structure has an antiparallel coupling (APC) layer between first and second ferromagnetic pinned layers. The first pinned layer has a magnetization oriented in a first direction perpendicular to the ABS by exchange coupling with the AFM pinning layer. The second pinned layer is antiparallel exchange coupled with the first pinned layer because of the selected thickness of the APC layer between the first and second pinned layers. Accordingly, the magnetization of the second pinned layer is oriented in a second direction that is antiparallel to the direction of the magnetization of the first pinned layer.
Another type of MR read element is a Magnetic Tunnel Junction (MTJ) read element. The MTJ read element comprises first and second ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetic moments, or magnetization directions, of the two ferromagnetic layers. In the MTJ read element, the first ferromagnetic layer has its magnetic moment pinned (referred to as the pinned layer). The second ferromagnetic layer has its magnetic moment free to rotate in response to an external magnetic field from the magnetic disk (referred to as the free layer). When a sense current is applied, the resistance of the MTJ read element is a function of the tunneling current across the insulating layer between the ferromagnetic layers. The tunneling current flows perpendicularly through the tunnel barrier layer, and depends on the relative magnetization directions of the two ferromagnetic layers. A change of direction of magnetization of the free layer causes a change in resistance of the MTJ read element, which is reflected in voltage across the MTJ read element.
GMR read elements and MTJ read elements may be current in plane (CIP) read elements or current perpendicular to the planes (CPP) read elements. Read elements have first and second leads for conducting a sense current through the read element. If the sense current is applied parallel to the major planes of the layers of the read element, then the read element is termed a CIP read element. If the sense current is applied perpendicular to the major planes of the layers of the read element, then the read element is termed a CPP read element.
One problem facing designers of read elements is stabilizing the magnetic moment of the free layer. Although the magnetic moment of the free layer is free to rotate upwardly or downwardly with respect to the ABS in response to positive and negative magnetic fields from the magnetic disk, it is important to longitudinally bias the free layer (biased parallel to the ABS and parallel to the major planes of the layers of the read element) to avoid unwanted movement or jitter of the magnetic moment of the free layer. Unwanted movement of the magnetic moment adds noise and unwanted frequencies to the signals read from the read element.
One current method used to stabilize the magnetic moment of the free layer is to bias the free layer using an in-stack biasing layer structure. The biasing layer structure includes a pinned bias layer and a pinning bias layer separated from the free layer by a spacer layer. The pinned bias layer has a magnetization pinned by exchange coupling with the pinning bias layer. The pinned bias layer is generally very thick, on the order of 40 Å or more. Because of the large size of the pinned bias layer, the pinning bias layer does not effectively fix the magnetic moment of the pinned bias layer permanently. Because the magnetic moment of the pinned bias layer is unstable, the pinned bias layer cannot effectively stabilize the magnetic moment of the free layer. Therefore, one problem with this configuration is that the pinned bias layer provides weak biasing of the magnetic moment of the free layer.
Another current method used to stabilize the magnetic moment of the free layer is to bias the free layer using first and second hard bias magnetic layers that abut first and second sides of the read element. There are multiple problems with this configuration. First, because the hard bias magnetic layers are on either side of the read element, side shields cannot be inserted on either side of the free layer of the read element. Secondly, the hard bias magnetic layers are insulated from the free layer and the rest of the read element. The insulation and the gap between the magnetic layers and free layer reduce the magnetic field applied to the free layer from the magnetic layers. The reduced magnetic field can provide for weak biasing of the magnetic moment of the free layer. Third, the hard bias magnetic layers do not uniformly bias the free layer. The end portions of the free layer can become over-biased and do not properly respond to magnetic fields from the magnetic disk.
Therefore, a need exists for a more effective way to bias the free layer of a read element.